In a typical postage meter, also called a franking machine, a horizontal surface defines a paper path along which travels a mail piece such as an envelope. As the mail piece moves along the paper path toward the print rotor, it trips a mechanical trigger. The trigger causes the print rotor to turn and the mail piece is received and printed upon. The mail piece passes between a print rotor and a platen roller to receive an imprint of postage.
Those familiar with postage meters will appreciate that the rotor does not rotate continuously, but performs single revolutions in keeping with the arrival of mail pieces. Even if the mail pieces move continuously, the rotor movement is discontinuous. The rotor is motionless, at least briefly, between each printing cycle. From the point of view of the rotor and its trigger, mail pieces may arrive uninterruptedly for some time and then may stop abruptly. The trigger is a crucial part of the system that causes the rotor to rotate at the right times, and lets the rotor remain motionless at the right times.
A descending register keeps track of the amount of postage available for printing, and when the descending register shows a too-small amount the meter locks up and cannot print any more postage. Each rotation of the rotor causes postage value to be deducted from the descending register, and for that reason it is highly desirable that the rotor only be actuated for rotation when a mail piece is in place to receive the postage imprint. Errors in either direction are troublesome. If the rotor rotates with no mail piece is place, then the user of the postage meter loses money. If a mail piece arrives and the rotor does not move, then this constitutes a jam that will have to be cleared.
The range of printing problems to be guarded against is much greater than simply printing postage when it is not needed, or failing to print postage. If a postage imprint is mispositioned this is also a serious problem. Relative to the right edge of an envelope, if the imprint is too far to the right then part of the imprint may be off the paper and result in a spoiled piece of mail. If the imprint is too far to the left, the imprint may overlap printed portions of the envelope such as the return address, or in an extreme case may stray past the left edge of the envelope. These problems are, at the very least, aesthetically displeasing, and can also result in loss of postage value or jams.
These problems present themselves in any postage meter, but are particularly troublesome in meters that are intended for high-speed use with as many as 10,000 pieces per hour receiving postage imprints. With such meters it is desired to have a trigger mechanism for the printing of postage that consistently prints postage in the desired position on the mail piece, and that can easily be set to predetermined configurations to accept different types of mail pieces. A traditional prior-art mechanical trigger does not work well with thin pieces, for example, air mail envelopes, and it can degrade mechanically and abrade.
One prior-art example of a trigger mechanism for use in a postage meter is U.S. Pat. No. 5,203,263, assigned to the same assignee as the present application and incorporated by reference. Another prior-art example is U.S. Pat. No. 4,523,523 to Abellana et al. A few other prior-art references mention postage meters and optical sensors, namely U.S. Pat. Nos. 4,840,696, 4,571,925, and 4,310,755.